1. Field of the Invention
The invention relates to the field of detection of radiation in a semiconductor substrate. More in particular, the present invention is related to a time-differentiated measurement of the spatial modulation of carriers generated by radiation and diffusing in the substrate.
2. Description of the Related Art
Devices receiving light at high speed in specially conceived receiver photo-diodes constructed in, for example, silicon or in III-V semiconductor material arc known in the art. The acquired information stream is often further processed or used in CMOS or BICMOS circuitry. This approach requires hybridization of the receiver (or of the receiver diodes) with the CMOS (or BICMOS) circuitry. This lowers system yield and adds significantly to the cost of the system. When constructing, for example, parallel optical interconnects between CMOS circuits, there is a need for light sources and for light detectors. Large numbers of detectors need to be connected to the CMOS circuitry, which is difficult to achieve. Detection of the light directly in CMOS avoids most problems associated to hybridization, including cost and reliability.
However, it is known that light detection directly in CMOS yields low detector-response times. As stated by Levine et al. in "1 Gb/s Si high quantum efficiency monolithically integrable 880 nm detector", Applied Physics Letters vol. 66 p.2984, 1995, a typical response time is 200 ns. The same publication shows operation at a higher frequency (1 Gb/s) by deviating strongly from standard CMOS concepts.
A typical problem when using the neutral zone in the semiconductor substrate for constructing a direct light receiver system is that the attainable bit error rate is bad (high) due to photo-generated carriers diffusing around in the substrate for as long as their lifetime, depending on previous light-input patterns. However, by operating differentially, it has been proven very recently by Ayadi et al.: "A Monolithic Optoelectronic Receiver in Standard 0.7-.mu.m CMOS Operating at 180 MHz and 176-fJ Light Input Energy", IEEE Phot. Techn. Lett., vol. 9, 88-90, 1997 that faster operation is possible using a mere CMOS based detection system. The solution being disclosed by Ayadi requires, however, two light input channels, which is not practical for many communication applications.
U.S. Pat. No. 4,096,512 discloses a light detector which employs two interdigitated PN junction light detectors, one of which is covered by an opaque material. The one covered by opaque material is used as a standard for eliminating dark current effects. This document teaches how to cancel the dark current of a photo-detector, but does not teach how to speed up photo-detector response. In order to eliminate insensitivity due to high "dark current", this document uses two interdigitated relatively large area PN junctions, one of which is covered by an opaque material. The result is that the two PN junctions will have closely matched dark currents one of which can be used as a reference for determining the current due exclusively to illumination on the other. As a result of the opaque layer, the leakage current measured between the two conductors is representative of the "dark current" for the device formed by the region uncovered and the substrate as well as for the device formed by the covered region and the substrate. The leakage current of the device formed by the covered region and substrate can be subtracted by an external circuit from the leakage current of the device formed by the uncovered region in the substrate. This means that there is an unmasked detector and a reference detector. The reference detector only has a dark current. The dark-current is generally known to be only function of temperature and metallurgical junction quality, and is hence a DC value (except for very slow variations with temperature). Using the teaching of this document, no faster detector response will be obtained by subtraction of the dark currents. Since subtracting with a DC-value does not change timing behavior, it only gives an offset.